THE AGRICULTURAL JOURNAL. 685 



AgHcutturai Chemistry for Beginners. 



CHAPTER IV. 

 By Archibald Pearcb. 

 NITRIC ACID AND THE NITRATES.— NITROGENOUS MANURES. 



ITRIC acid is chiefly noticeable for its 

 ri salts, the nitrates ; for it is in the 

 form of nitrates that plants obtain almost, 

 if not quite, all of their nitrogen. It is a 

 liquid compound of hydrogen, nitrogen, 

 and oxygen, very corrosive and poisonous, 

 formed by the combination of an oxide of 

 nitrogen with water, and is manufactured 

 from a nitrate. It is a very striking in- 

 stance of the way in which chemical com- 

 pounds differ from their constituent 

 elements, that the harmless gases of the 

 air, first combining together and then 

 with water, should produce the deadly 

 poisonous and destructive nitric acid. 

 The nitrates are almost entirely the result 

 of the action of a microbe in the soil 

 which has the power of converting am- 

 monia into the nitrate of any base that 

 may be present, usually lime, and so 

 fitting it for the use of plants. This con- 

 version takes place most readily in hot 

 climates such as India, where, owing to 

 the large amount of decomposing animal 

 and vegetable matter in the ground, there 

 is always a quantity of ammonia, and, 

 consequently, of nitric acid, in process of 

 formation : and this is so marked that 

 nitrate of potash, or saltpetre, of ten occurs 

 in considerable quantities in the soil, and 

 is washed out and purified for sale. The 

 chief conditions which favour the process 

 ( f nitrification, as it is called, are warmth, 

 air, moisture, and the presence of phos- 

 phates and of some base for the nitric 

 acid to combine with. If we look into 

 these conditions it will be plain that they 

 are best fulfilled in a well-drained and 

 wel^tilled soil, for if the land is sodden 

 and water-logged, or if it is packed hard 

 and not -well Lroken up, the air cannot 

 get entrance. Moreover, it is probable 

 that in the absence of air, not only do the 

 nitrifying organisms fail to flourish, but 

 another variety cemes into action which 

 actually undoes the useful work of the 

 others, decomposing the nitiates and set- 

 ting free nitrogen, which, as we know, is 



useless to most plants in its uncombined 

 state. It is easy to see, therefore, how 

 necessary it it is, for good results, that the 

 land should be kept well stirred and 

 drained if the crops are to receive the 

 nitrogenous nourishment that is so indis- 

 pensable for their well-being. 



The nitrates are all very soluble, and 

 not strongly retained by soils, so that they 

 are easily washed away by heavy rains ; 

 for this reason, except where the rainfall 

 is light, if used as a manure they are best 

 applied as a top-dressing. This excessive 

 solubility explains the ditficulty there 

 often is in keeping up the supply of nitro- 

 gen in arable soils without the applica- 

 tion of nitrogenous manures, for there 

 will be a gradual loss of nitrates due to 

 soakage, which can only be partially re- 

 placed from natural sources. A small 

 quantity of nitric acid is often formed in 

 the air by the passage of lightning dis- 

 charges through it, causing the oxygen 

 and nitrogen to combine, and then form 

 nitric acid with the water present. This 

 then further unites with any ammonia 

 that may be in the air, forming am- 

 monium nitrate, which is brought down 

 to the earth by the rain, and acts as a 

 small supply of plant food. 



Nitrate of Potash. 



The two most important nitrates are 

 nitrate of potash, also known as nitre or 

 saltpetre, and nitrate of soda. The former 

 has been manufactured on a considerable 

 scale in the so-called nitre-beds. A pile 

 is made of earth, wood-ashes, and any 

 animal or vegetable refuse available, kept 

 moist by watering with the drainage of 

 stables or cattle slieds, but covered to pro- 

 tect it from excessive wetting by rain. 

 The mass is occasionally turned over to 

 expose it to the air, antl after a time large 

 quantities of saltpetre are formed by 

 the action of the nitrifying microbes, and 

 this is collected and purified. A similar 

 action goes on in a compost heap or an 



